Hi there, jmorris. 

You can see the invoice payment method through the payment transaction created. I'll show you how.

1. Open the invoice.
2. Tap on the payment link under the Paid status to see the payment created.
3. You can review the payment method under the Payment method section. If you wish to modify it, select another option from the drop-down menu.

If you want to add the payment method to your invoice, you can manually input it under the Description column.

For more tips about handling your invoices and payment transactions, you can open this article: Record invoice payments in QuickBooks Online.

You can also get more tips about handling your sales and other customer entries through this link: Topics about your company's income and customers.

If you have any other follow-up questions about an invoice payment method, let me know by adding a comment below. I'm more than happy to help. Keep safe!